This invention relates to double wall metal pipe, and more particularly, to double wall helical pipe formed of a single elongated strip, to the method of its formation and the configuration of metal strip utilized in such formation.
Pipe as herein described may be formed to have two smooth walls, two corrugated walls or one smooth and one corrugated wall, and a highly desirable form of such double wall pipe will have a smooth inner wall and a corrugated outer wall. The pipe can be used in place of conventional corrugated pipe for underground drains, culverts, and the like, or for efficient transfer of liquids, air or other gases, and in such applications the smooth inner wall will avoid problems associated with a corrugated surface such as poor drainage from corrugation valleys and, during cold weather, formation of ice in the corrugations. A corrugated surface in a liquid conveying pipe causes interference with and turbulence in the flow of liquid. Pipe having a corrugated outer wall and smooth inner wall avoids the problems associated with a corrugated interior surface and at the same time exhibits increased strength relative to single wall pipe of corresponding wall thickness. Double wall construction lends itself to the use of special purpose insulating or corrosion resistant materials for the inner wall or layer between walls.
Known forms of double wall metal pipe are fabricated from two sheets, strips or tubes of metal rather than from a single strip. For example, in the method for forming laminated pipe disclosed in U.S. Pat. No. 3,487,537, Lombardi, two metal strips are advanced along two separate forming mills and are curled together to form helical pipe. Forming double wall pipe from a single strip avoids the need for precisely aligning and coordinating manipulation of two separate work pieces. This is a significant advantage as it is very difficult to effect accurate control of metal strip during pipe forming, especially in the case of strip material for forming heavy wall pipe having a thickness in excess of one-eighth inch. Simplified forming equipment may be utilized when forming from only one strip since, for example, the method obviates the need for two separate strip coil reels and mills such as disclosed in the method of U.S. Pat. No. 3,487,537.